Tracking down the lead actor in Birdwatchers is no easy task, but then Ambrósio Vilhava is not your typical Brazilian movie star. Dozens of calls to his mobile phone are diverted straight to his answering machine. Dozens more go unanswered. One time a child's voice answers before the line goes dead. Then, finally, on the eighth day of trying, his heavily accented Portuguese crackles over the line.

Vilhava is not a soap-opera heart-throb or a musclebound action hero. He is a Guaraní-Kaiowá Indian and grandfather of two who lives in a tiny village in Brazil's midwest and was catapulted on to the country's big screens with his performance in Italian director Marco Bechis's new film, Birdwatchers. "It wasn't difficult to act, it was my dream," says Vilhava, who plays Nadio, a troubled Kaiowá leader fighting to regain his ancestral lands. "I always dreamed that one day we could show what our situation is like and tell the story of all of our relatives who have been murdered, mistreated and who have never seen justice." Bechis's film is both a devastating portrait of Brazil's indigenous populations in the 21st century and a tribute to the resistance of the Guaraní-Kaiowá, who are gradually fighting back against the ranchers and farmers who have driven them off their traditional lands. "The Guaraní-Kaiowá are survivors – survivors of ancient times," says Bechis.

When Bechis touched down in Brazil in April 2004, he came expecting to make a film about the Indians of the Amazon rainforest: remote and ancient tribes who wielded bows and arrows and lived in straw huts hidden deep in the jungle. "I wanted to do a film about indigenous people in Latin America. But [like] every romantic European, I was supposed to do a film about the Amazon," says Bechis.

Instead, acting on the advice of members of indigenous rights group Survival International, whom he had met in Milan, Bechis travelled to the Brazilian state of Mato Grosso do Sul. There he found Vilhava and the Guaraní-Kaiowá people struggling to regain their land, thousands of miles south of the world's largest tropical rainforest. "When I met the Kaiowá just dressed in normal clothes, drinking alcohol and using mobile phones – even if they were never charged up because they don't have electricity – I said: 'This is what Indians are today. This is the film.'"

Shot over 10 weeks in 2007, Birdwatchers focuses on a group of Guaraní-Kaiowá as they attempt to reclaim their land from a local farmer, in what is known in Portuguese as a retomada or "retaking". While the wealthy cattle rancher kicks back on his idyllic country estate, showing European birdwatchers slides of the local wildlife, the Guaraní-Kaiowá eke out a precarious existence, slaving away on sugar cane plantations and living in a shabby squatter settlement.

For Bechis, who was born in Chile, raised between Buenos Aires and São Paulo, and moved to Italy after being expelled from Argentina in 1977, Birdwatchers fulfilled a long-held desire to portray the lives of the Indians of South America. "At school [in Buenos Aires], we didn't talk about the problem [of the plight of indigenous peoples]. Yes, we knew all about the Aztecs, the Mayas and all the ancient cultures but we didn't study anything about the conquest, about what happened after 1500 in Latin America. The indigenous problem was completely forgotten. There was a hole."

According to anthropologist Rubem Thomas de Almeida, one of Brazil's leading Guaraní specialists, there are currently around 60,000 Guaraní living in Brazil and around 45,000 in Mato Grosso do Sul. Almeida claims that some archeological studies show they may have been in the region since the fourth century. But as Brazil's agricultural frontier advanced throughout the last century, the Guaraní were shunted off their lands by tea planters, sugar-cane farmers and, more recently, soy growers. Today, indigenous leaders such as Vilhava continue to fight for the right to return to such areas.

While films such as City of God, Elite Squad and Hector Babenco's Carandiru have forced Brazilian cinema audiences to sit up and take note of the country's urban crises, the often desperate plight of Brazil's indigenous populations has remained largely in the shadows. Birdwatchers attempts to fill this hole, documenting the contradictory lives of the Guaraní-Kaiowá, who appear engaging in age-old traditions one moment and then intoxicating themselves on potent Brazilian rum or listening to techno music the next.

According to Survival International, 517 Guaraní-Kaiowá have committed suicide over the last 20 years, among them a nine-year-old boy. These deaths are blamed partly on the dismal living conditions the Kaiowá are forced to endure, crowded into government reserves or shanty towns on the outskirts of the region's cities. Earlier this month, Amnesty International called for urgent action to prevent a group of 130 Kaiowá, including 60 children, being evicted from their ancestral land in the same region. "The situation of the Guaraní-Kaiowá tribe is one of the most serious in the Americas," says Fiona Watson, Survival's Brazil campaigner, who was involved in setting up contact between Bechis and the Kaiowá. "Despite being one of the largest tribes in Brazil, they have almost no land. Almost every community has experienced violent eviction from their land, which has resulted in shockingly high rates of suicide and malnutrition as the Indians see no future."

As a real-life Kaiowá leader, whose family was evicted from its ancestral land in the 1950s by cattle ranchers and tea planters, Vilhava understands the film better than most. He explains the importance of ancestral land to the Guaraní-Kaiowá. "The white man is born in the hospital and his umbilical cord is thrown in the rubbish bin," he said. "The Indian is different. He is born on the ground and his umbilical cord is buried in the earth. This is what a retomada is – the return of the Indian to his land because that is where the secret of the Indian is."

Birdwatchers began to take shape in 2005 when Bechis began making contacts with the Kaiowá, with the help of a local human rights lawyer, Nereu Schneider. Bechis then spent two months living with Vilhava before returning to Europe in search of funding. "It was very hard to get money to do it because we had no stars," he recalls. "Producers said to me: 'You will never finish the film if you use Indians as actors.'" It took two years, but by 2007, Bechis had his funding and began the casting process, inviting 200 indigenous people, men, women and children to tests, where they were shown films by Sergio Leone, Pier Paolo Pasolini and Alfred Hitchcock. Vilhava says when he first told other Guaraní that he wanted to make a film, he got the response: "You're aiming very high, don't you think?"

"But," he says, "when I was a child, I heard people talk about films, I heard the white men talking about cinema. I wanted to know what it was. Then later I saw a television and I thought I should know what this was. It looked difficult."

When the shoot began, in September 2007, the challenges of working with first-time actors, many of whom had never set foot inside a cinema, became immediately apparent. "They have an oral tradition, so it was impossible to give them a script," says Bechis, who instead held early morning meetings each day at which the Indians were consulted on the most realistic way of recording each scene. After the meetings, "often the scene changed completely".

Birdwatchers' Brazilian scriptwriter, Luiz Bolognesi, sat at "a table, with a little chair in the middle of the jungle rewriting his scenes on a laptop." But Bechis believes working with untrained performers gave the film a realism that would have been impossible with non-indigenous actors. Often Vilhava or other cast members would approach him and say: "Listen, Marco: I can do it like you want – but I wouldn't do it like that."

According to Almeida, the anthropologist, the result is "a very perceptive film about how the Guaraní are. The film shows what is real: the boozing, the suicide but in a very Guaraní way," he says. "It is a fiction that is absolutely linked to reality." Indigenous rights activists, such as Watson, whose group has launched an international campaign to support the Guaraní to coincide with Birdwatchers' release, hope the film will be more than a box-office success. "We hope that Birdwatchers, in which the Guaraní themselves act out their story, will inspire people to support them," she says. "The Guaraní's plight has been ignored for decades by successive Brazilian governments yet this tragic situation could be easily reversed if the government acts now to recognise their land rights. Public opinion can help."

Exactly two years on from the shoot, Birdwatchers may have launched an improbable acting career for Vilhava, who says he is currently considering an offer to appear in a Russian film about the Guaraní-Kaiowá. But, most important, Birdwatchers has put his long-forgotten people on the map, he says. "The Indians didn't come from the other side [of the world] to be here. The Europeans did. So how can they call the Indian an invader? The film shows the reality of the Guaraní-Kaiowá. What was under the table, is now on top of the table for everyone to see."

Birdwatchers is released next Friday.

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